Injection molding is a manufacturing technique for making parts from plastic and other materials where molten material (e.g., plastic, metal, ceramic, etc) is injected at high pressure into a mold that is the inverse of the desired shape. The mold is typically made by from metal, usually either steel or aluminum, and precision-machined to form the features of the desired part. Once the molten material cools within the mold, the mold is opened (e.g., first and second mold portions are separated along a mold parting line) and the molded part is removed.
Injection molding is widely used for manufacturing a variety of parts, from very small parts (e.g. cell phone cases, micro watch parts etc.) to large parts (e.g., body panels of cars). Injection molding is accomplished by injection molding machines that are operative to compress mold portions together and inject molten material into the compressed mold portions (i.e., that define a desired shape/part). Typically the mold is held together (i.e. closed) by a mechanical force. The clamping force necessary to maintain the mold portions together during the injection process is defined as the injection pressure multiplied by the total projected area of the mold. In cases where two or more molds are stacked, the necessary clamping force is defined by the injection pressure times the greater projected area of the two or more aligned molds.
In order to provide molten material to the mold, injection materials, typically in the form of granular pellets, powders, strips, and in some instances liquids, are fed to the machine through a feed throat. In one arrangement, the injection materials enter an injection barrel by gravity though the feed throat. Upon entrance into the barrel, the material is heated to an appropriate temperature.
Typically melted material (i.e., melt) is injected into the mold by a positive or semi positive linear device such as a reciprocating screw or a ram injector (e.g., a hydraulic injector). In a system utilizing a reciprocating screw, the injection material (e.g., pellets, powder, strips, liquid etc.) moves forward as the screw rotates and undergoes extreme pressure, heat and friction that generates most of the heat needed to melt or the injection material. Heaters surrounding the screw may also assist in the heating and temperature control of the injection material during the process.
The mold is the part of the machine that receives the injection material and shapes it appropriately. The mold is controlled constantly to a temperature that allows the material to solidify sometimes at a reduced temperature. Fluid-cooling channels and cooling fluid may assist in cooling the material. The cycle is completed when the mold is opened and the part is ejected (sometimes with the assistance of ejector pins within the mold).
The number of parts that may be produced by an injection molding machine is dependent on the cycle time of the process. The cycle time is determined by the time required to inject the molten injection material into the mold, solidify the injection material, open the mold, eject the part(s) and close the mold. It will be appreciated that a single mold may include a plurality of cavities to permit, for example the molding of a plurality of identical parts. In this regard, a single set of parts may be generated during each cycle. To increase the productivity of an injection molding machine, stacked molds may be utilized. Such stacked molds generally are formed having two or more parting lines defining separate sets of mold cavities. These separate sets of mold cavities that often have aligned projection areas such that both sets of molds may be filled without an increase in the clamping force. Such stacked parting line molds allow for two or more sets of parts to be manufactured during a single cycle.
One drawback to the use of stacked parting line molds is that the molds must be specially designed to allow for transfer of injection material from the first mold defined by a first parting line to a subsequent mold. For instance, heated runners (i.e., feed channels) may be required to allow for such transfer between molds. Such specialized molds require added expertise in design and manufacture, require additional space in an injection machine and increase the cost and lead time to procure the molds.